If you are using a custom field in a SAEF, then what you have is correct IF indeed the textarea field has an id of ‘6’ and you follow it up with {field_id_6}</textarea>. If you are using an SAEF with form_helper plugin, you have to have it like:
<textarea name=”{exp:form_helper:field_grabber field_name=‘field_id_6’}” rows=“20” cols=“70”>{exp:form_helper:field_parser}{field_id_6}{/exp:form_helper:field_parser}</textarea>
although field_parser tag is optional.
You can also use the short name for the field. The only exception is if you are using the {custom_fields} call and you have multiple textarea custom fields. I’m starting to guess where I would have to see more code or understand the weblog structure you are implementing to be of more use.
Also, I did find out in messing with a Javascript validation on input fields, that different browsers (specifically IE in my case) interchanges the ID with the NAME input attribute if one isn’t there, where others do not. Just to be safe, put both ID and NAME makeing them equal to the same value to test.